The problem with distributed reflectometry methods is that the measurement made by a reflectometer is affected by parasitic test signals from the other reflectometer or reflectometers. This interference becomes more limiting if the various reflectometers making up the distributed system are not temporally synchronized with one another.
The received signal must therefore be subjected to special processing in order to eliminate this interference; if this is not done, the measurement cannot be used. This processing consists in discriminating the signals originating from different reflectometers in order to retain only the signal emitted by the reference reflectometer.
A first known method of distributed reflectometry is described in French patent application FR2907910, filed by the present applicant. This method consists in using pseudo-random sequences of the M-sequence type as a test signal. The aim of the method is to minimize cross correlation, that is to say the statistical resemblance of signals, between the sequences generated by the different reflectometers. The post-processing performed on the measurements of the reflected signals consists in applying appropriate filtering, as is done in the known method of reflectometry called sequence time domain reflectometry (STDR). A drawback of this method is that it requires post-processing to discriminate the measurements corresponding to the signals injected by the different reflectometers. Furthermore, the profile of the test signal concerned is not particularly suitable for diagnosing long cables (several kilometers in length), since the attenuation of the injected signal affects the performance of the discrimination algorithm.
A second method of distributed reflectometry forms the subject of international patent application WO2010/043602, filed by the present applicant. This method uses a set of weighting coefficients for modulating the test signal and a weighted average of the measurements of the reflected signal. The weighting coefficients are chosen so as to be orthogonal to one another, so that only the signal originating from the desired sources is retrieved, the interference noise from the other sources being canceled by the calculation of the average.
However, this method is valid only if the injected test signal has a zero average, which cannot be the case, for example, with a temporal pulse.
This is because, if a full scale pulse is injected into a very long line, that is to say if the whole dynamic range of the converters is used, including the range on the negative voltage side, then a disturbing transition is observed at the changeover point of the weighting coefficient, due to the effects of capacitive discharge. This transition interferes with the average that is calculated subsequently, and therefore with the final measurement of the reflectometer.